The patent to Ironfield, U.S. Pat. No. 3,182,164, issued May 4, 1965, entitled "Electromagnetic Energy Seal," discloses a microwave oven having an oven door with a radiation for inhibiting the leakage of microwave energy from the oven around the periphery of the door. The seal utilizes an input section from the oven in which the door conductive wall is spaced from the oven conductive wall and constitutes a transmission line from the oven interior to a region of coupling to a branch transmission line coupled to the input section at a point approximately a quarter wavelength at the microwave energy supply frequency along said input transmission line section from the oven end of said input section. The branch transmission line is shorted by a conductive end plate at a point approximately a quarter wavelength from the coupling to the input transmission line and, hence, reflects a short circuit to the oven end of the input transmission line section at a distance of one-half wavelength from the shorted end of the branch transmission line.
Such a structure may be also considered as an open circuit or high impedance reflected from the shorted end of the branch transmission line to its point of junction with the main transmission line, such open circuit being in series with one of the walls of the input transmission line at the point of junction, hence inhibiting the further transmission of energy along the transmission line and out of the oven. This reduces the electrical effect of various structures connected to the output region of the transmission line or the outer peripheral portion of the door.
The peripheral impedance discontinuities, as shown in U.S. Pat. No. 3,767,884 issued Oct. 23, 1973 to John D. Osepchuk and James E. Simpson, prevent peripheral transmission of energy in the seal and permit use of transmission line dimensions which can be accurately matched to the microwave frequency since dimensions transverse to the seal periphery predominantly control the resonant or impedance mismatch of the seal structure.
In production, the metallic parts of the door are preferably fabricated as die formed parts, such as metal stampings or castings, and die tolerances will produce variations in the resonant frequencies of the choke action which for high Q materials may cause some of the oven seals to fall outside allowable radiation standards. In addition, when ovens are in service, bending of the door hinge or other parts can cause conditions where leakage is beyond standards. Hence, it is desirable to set the production run so that leakage is minimized for the worst case to be encountered. An example of a worst case is the condition when a door is slightly opened with the oven energized before an interlock deenergizes the power supply. Since changes in the dies for stampings or molding of metal or plastic are economically undesirable, it is desirable to have another means of altering the electrical wavelength of the seal on a product run basis.